is a city and municipality in Østfold county Norway. The administrative centre of the municipality is the city of Sarpsborg. Sarpsborg is part of the fifth largest urban area in Norway when paired with neighbouring Fredrikstad. As of 1 January 2010 according to Statistics Norway these two municipalities have a total population of 125 797 with 52 159 in Sarpsborg and 73 638 in Fredrikstad. Borregaard Industries is and always has been the most important industry in the city.